This is a solid piece of work that feels like an authentic gbc game,with nice graphics for the simple gbc feel, decent music which could use a little more variety, but is appropriately intense. The gameplay is solid, though I do have a little constructive criticism to bring:
Having the player suddenly respawn to the far left when they take a hit and lose a life is a bit jarring; I think it's better to just leave them where they are as the invincibility works decently to protect a player, and perhaps the life icons could have a quick destructive animation (Just say 2-3 frames) so the player catches sight and recognizes they just got hit.
I think the player could be a hair faster, just by a tiny, tiny,tiny hair, as it's not too badly balanced now, but fine tuning it a little could help in the long run to feel a bit better.
Having enemies spawning instantly at doorways leads to annoying instant deaths, especially for the left,right and bottom doors which have no real indication that they are opening, which leads to some very cheap deaths.
Starting each level with exactly how many life points you had when completing he previous floor is a touch too frustrating, as continually restarting a floor with just a single health or two tends to grind a bit much.
I suggest having the player receive at least one life upon completion of a floor, perhaps at a cap of 3 and no more, only allowing the player to gain more life through those small green powerups.
Critique aside, this is quite fun and it has some good promise.